Abstract
Neoproterozoic banded iron formations (BIFs) are not restricted to the middle Cryogenian, c. 715 Ma glaciation, occurring in Tonian, Cryogenian, and Ediacaran successions. Many Neoproterozoic BIFs were deposited in glacially influenced settings, such as the Rapitan Group (Canada), Jacadigo Group (W Brazil), Chuos, and Numees Formations (Namibia) and Holowilena Ironstone (Australia). However, many occurrences are not related to glacial processes and can be assigned to the Algoma and Lake Superior types. Neoproterozoic Algoma-type BIF includes the Wadi Karim and Um Anab (Egypt), the correlative Sawawin BIF (Saudi Arabia), and the Jucurutu Formation of the Seridó Belt (NE Brazil). Lake Superior type BIFs are represented by the Tonian Shilu Group (South China) and the late Ediacaran Arroyo del Soldado Group (Yerbal and Cerro Espuelitas formations, Uruguay).
Useful chemostratigraphic tools for the study of BIFs include rare earth element distribution, especially Eu and Ce normalized concentrations, iron speciation, and Nd and Cr isotopes (δ53Cr). Whereas Rapitan type BIFs exhibit no Eu or Ce anomalies, the Algoma-type Neoproterozoic BIFs show both. Positive δ53Cr values characterize glacially influenced BIFs, and differentiates them from nonfractionated, mantle-like δ53Cr values of Algoma type BIF. The most positive δ53Cr values occur in the open-shelf, Lake Superior type BIF, especially that of Ediacaran age.
We suggest that glaciation was not a prerequisite for deposition of BIF in the Neoproterozoic, and that hydrothermal activity related to Rodinia rifting played the key role in their reappearance after the Mesoproterozoic gap. Their deposition has more to do with the return to ferruginous conditions in the oceans than with near-global glaciation.
Useful chemostratigraphic tools for the study of BIFs include rare earth element distribution, especially Eu and Ce normalized concentrations, iron speciation, and Nd and Cr isotopes (δ53Cr). Whereas Rapitan type BIFs exhibit no Eu or Ce anomalies, the Algoma-type Neoproterozoic BIFs show both. Positive δ53Cr values characterize glacially influenced BIFs, and differentiates them from nonfractionated, mantle-like δ53Cr values of Algoma type BIF. The most positive δ53Cr values occur in the open-shelf, Lake Superior type BIF, especially that of Ediacaran age.
We suggest that glaciation was not a prerequisite for deposition of BIF in the Neoproterozoic, and that hydrothermal activity related to Rodinia rifting played the key role in their reappearance after the Mesoproterozoic gap. Their deposition has more to do with the return to ferruginous conditions in the oceans than with near-global glaciation.
| Original language | English |
|---|---|
| Title of host publication | Chemostratigraphy : Concepts, Techniques, and Applications |
| Editors | Mu Ramkumar |
| Publisher | Elsevier |
| Publication date | 19 Feb 2015 |
| Pages | 433–449 |
| Chapter | 17 |
| ISBN (Print) | 978-0-12-419968-2 |
| ISBN (Electronic) | 9780124199828 |
| DOIs | |
| Publication status | Published - 19 Feb 2015 |